Mucosal atrophy often leads to barrier failure and sepsis in starving or fasting patients in spite of total parenteral nutrition (TPN). In addition, mny patients who undergo massive bowel resection fail to adapt and must be maintained on permanent TPN with high mortality. Current therapies aimed at preventing mucosal atrophy and enhancing adaptation are limited in efficacy, perhaps because they mostly induce proliferation. We propose that we must learn to promote differentiation as well as proliferation to achieve an intact functional mucosa, reverse mucosal atrophy, and engender maximal intestinal adaptation. Some Schlafen-family proteins mediate cell growth, differentiation, or development in some cells in a Schlafen- and cell-specific fashion. We have demonstrated that the rodent protein Schlafen-3 (Slfn3) is induced during and required for intestinal epithelial differentiation in response to various stimuli. Our data indicate that Slfn3 varies with mucosal atrophy and promotes differentiation in vivo and that Schlafen-12 (SLFN12) is its human ortholog. We hypothesize that Schlafen 3/12 induction represents a fundamental and essential common pathway for enterocytic differentiation that can be specifically targeted to promote differentiation and maintain the mucosa. We further propose that Schlafens 3/12 are chaperoned to the nucleus where they enhance differentiation by affecting transcription and by a Cdx2 homeobox protein- dependent positive feedback mechanism. We will study intestinal mucosa in patients fasting on TPN (atrophy) or after Roux-en-Y gastric bypass (hypertrophy), rat and human intestinal epithelial cells in vitro, a novel rodent model of in vivo intestinal epithelial atrophy and hypertrophy, in vivo viral Slfn3/SLFN12 infection, and a tet-inducible intestine-specific Slfn3 knock-out mouse to achieve the following specific aims: 1) show that SLFN12 mediates human enterocytic differentiation and mirrors Slfn3 changes with differentiation in mucosal atrophy or hypertophy, 2) determine the mechanism by which Slfn3 and SLFN12 promote differentiation in rat and human intestinal epithelial cells, and 3) demonstrate that manipulating Slfn3/SLFN12 or a nuclear chaperone modulates intestinal atrophy/adaptation in vivo. This study will fill a critica knowledge gap because we will delineate a fundamental mechanism by which diverse stimuli regulate intestinal epithelial differentiation. This may facilitate interventions to maintain the mucosal barrier in starving or critically ill patients with mucosal atrophy and promote nutrition i pediatric or adult short bowel syndrome. The Slfn3 knock-out mouse will provide valuable insights not only into the convergent differentiation regulatory pathway we will study but also int normal gut development, mucosal healing and other intestinal disorders such as IBD. In addition, understanding how Schlafen proteins promote differentiation will have broader implications for differentiation and development in other epithelial tissues.